The present invention relates to a beam splitting system for splitting a light beam into a plurality of light beams and an optical information read/write device (for reading/writing optical information from/to an optical disc) that employs the beam splitting system.
In optical information read/write devices which read/write information from/to an optical disc, a laser beam is generally incident on the optical disc (record medium).
The data density of optical discs is increasing more and more in recent years and a variety of optical record mediums according to various standards are being proposed. Conditions for raising the data density of an optical disc may include: shortening of the wavelength of a laser diode (semiconductor laser) as the light source, increase of the NA (Numerical Aperture) of the object lens which converges the laser beam on a pit formed on the optical disc, etc. Satisfying the two conditions make it possible to irradiate the optical disc with a beam spot of a smaller diameter compared to the cases of conventional devices, by which information processing can be carried out even when smaller pits are formed on the optical disc, that is, high data density of the optical disc can be realized.
However, as the wavelength of the laser diode gets shorter and the NA of the object lens gets higher as seen in recent years, the convergent angle of the laser beam focusing on the recording surface of the optical disc is getting larger and ill effects of various aberrations become more intense. Consequently, it becomes difficult to apply the beam spot to a pit formed on the optical disc with high accuracy.
The thickness of the cover layer (protective layer) on the data recording surface of the optical disc varies from disc to disc, and the cover layer thickness may fluctuate even in a single optical disc. Therefore, wavefront aberration (mainly, spherical aberration) is caused by the variation/fluctuation of the cover layer thickness on the data recording surface and the quality of reproduced signals is necessitated to be deteriorated.
As a countermeasure against the problem, an optical information read/write device that detects the spherical aberration occurring on the optical disc surface and actively corrects the spherical aberration (caused by the aforementioned variation/fluctuation of the cover layer thickness and manufacturing error of lenses) by use of a spherical aberration correction optical system such as an expander lens has been proposed and has become widely known. An example of which is disclosed in Japanese Patent Provisional Publication No. P2002-150569.
However, in the case where the spherical aberration is detected by use of a photoreceptor, etc., a signal representing the spherical aberration to be detected can hardly be distinguished from the focusing error signal, and subsidiary criteria become necessary for detecting the spherical aberration signal and the focusing error signal making a clear distinction between the signals. In conventional techniques, for example, the spherical aberration correction and focusing operation are carried out based on measurements of moving distances of the object lens and a lens group of the spherical aberration correction optical system, or the spherical aberration correction is conducted based on a previously measured profile (thickness errors, etc.) of the optical disc (e.g. Japanese Patent Provisional Publication No. P2002-150598).
However, the employment of the subsidiary criteria for distinguishing between the spherical aberration signal and the focusing error signal as above (setting parameters for the criteria, etc.) is a troublesome task for the manufacturer. Further, the spherical aberration correction process by reading out the previously measured profile of the optical disc does not include a real-time measurement of the spherical aberration. Therefore, the method can not properly cope with wavelength variations of the laser diode, etc. in cases where the optical information read/write device is susceptible to aberrations due to its short-wavelength laser diode and high NA object lens, by which the spherical aberration correction and focusing operation can not be carried out correctly and precisely.